1. Field of the Invention
The present invention relates to a two-color holographic recording apparatus.
2. Description of the Related Art
A holographic memory system is known as a digital information recording system which applies the principles of holography. The information recording system is characterized by recording information signals recorded on a recording medium as optical signals. For the recording medium, photorefractive crystals such as lithium niobate (hereinafter abbreviated as xe2x80x9cLNxe2x80x9d) single crystals are used.
FIG. 1 shows a conventional holographic recording apparatus. Laser light 12 emitted from a laser light source 11 is split into signal light 12a and reference light 12b in a beam splitter 13. The signal light 12a is expanded in its diameter by a beam expander 14, and then irradiated to a dot matrix panel 15 as collimated light. The dot matrix panel 15 receives recording data, converted by an encoder, as electric signals, and forms a light and dark dot pattern on a plane. The signal light 12a including data signal components is optically converted as it transmits the dot matrix panel 15. The signal light 12a including dot pattern signal components passes through a Fourier lens 16 which Fourier transforms dot pattern signal components of the signal light 12a which is then converged into a recording medium 10. On the other hand, the reference light 12b split in the beam splitter 13 is guided to the recording medium 10 by mirrors 17, 18, and intersects an optical path of the signal light 12a within the recording medium 10 to form a light interference pattern. The recording medium 10 made of photorefractive crystal records the spatial intensity modulation represented by the light intensity of the light interference pattern as changes in refractive index. In the foregoing manner, the information is recorded.
During reproduction, on the other hand, the dot matrix panel 15, for example, blocks the optical path of the signal light 12a so that the recording medium 10 is irradiated only with the reference light 12b. Diffraction light, which reproduces the recorded Fourier-transformed signal light pattern, appears on the opposite side of the recording medium 10 irradiated with the reference light 12b. This Fourier-transformed signal light pattern is guided to an inverse Fourier lens 19 to inverse Fourier transform the signal light pattern to produce a dot pattern signal. The dot pattern signal is received by a light receiving element such as a CCD 20 which again transduces the dot pattern signal to an electrical digital data signal which is then sent to a decoder for reproducing the original data.
In the recording system as described above, since the recording medium is irradiated with the same reference light during reproduction as well as during recording, the recorded signal is erased simultaneously with its reproduction (reproduction induced deterioration).
A two-color holographic recording system is a system which prevents the reproduction induced deterioration. Specifically, this system is provided with a gate light source in combination with a light source in a conventional holographic recording system, and allows signals to be recorded only in a portion of a recording medium which is optically stimulated to a metastable state by gate light emitted from the gate light source. When signals are reproduced, the gate light is blocked so that the recording medium is irradiated only with reference light to read out the signals, so that the recording medium is not stimulated to the metastable state, thereby preventing the reproduction induced deterioration. The two-color holographic recording is described, for example, in D. von der Linde et al., xe2x80x9cMultiphoton photorefractive process for optical storage in LiNbO3xe2x80x9d, Appl. Phys. Lett., Vol. 25, pp 155-157 (1974). Recording media suitable for use in the two-color holographic recording are photorefractive crystals which may form bipolarons. For example, H. Guenther et al., xe2x80x9cIntensity dependence and white-light gating of two-color photorefractive gratings in LiNbO3xe2x80x9d, Opt. Lett. Vol. 22, pp. 1305-1307 (1997) describes LiNbO3 doped with Pr (praseodymium), while H. Guenther et al, xe2x80x9cTwo-color holography in reduced near-stoichiometric lithium niobatexe2x80x9d, Appl. Opt. Vol. 37, pp. 7611-7623 (1998) describes crystals which are produced by reducing LN crystals having a near-stoichiometric composition (which do not include impurities or slightly includes Fe).
In the two-color holographic recording system, even after information has been recorded, charge carriers optically stimulated to the metastable state within photorefractive crystals serving as a recording medium do not instantaneously return to the base state (or stable state) but gradually return to the base state. Such charge carriers at the metastable state are drifted and rearranged by a spatial electric field including information signals, and spontaneously form a spatial electric field which cancels the former spatial electric field. The formation of the mutually canceling spatial electric fields results in a relatively reduced difference in changes of refractive index in a light interference pattern which has been recorded as the difference in changes of refractive index. In this case, a diffracted signal light pattern reproduced from the recording medium exhibits reduced contrast, and therefore digital data signals eventually reproduced as electronic signals may include a number of error signals.
It is therefore an object of the present invention to provide a two-color holographic recording apparatus which is capable of reducing error signals possibly including in reproduced digital data signals.
The two-color holographic recording apparatus according to the present invention is adapted to impinge interferable signal light and reference light to a recording medium, which has been irradiated with gate light to be optically stimulated to a metastable state, to record information signals carried by the signal light. The apparatus comprises first, second and third light shutters for passing and blocking the gate light, the signal light and the reference light incident on the recording medium; and control means for controlling the respective states of the first, second and third light shutters.
Since the two-color holographic recording apparatus has the means for blocking the gate light in consideration of a time required for charge carriers within photorefractive crystals, which serves as a recording medium, to return from the metastable state to the base state, there are no charged carriers existing at the metastable state after information has been recorded. It is therefore possible to record a light interference pattern on the recording medium as a relatively large difference in changes of refractive index corresponding to the spatial intensity modulation of the light interference pattern including the information signals.